1. Field of the Invention
The present invention relates to a nuclear fuel assembly and more particularly to a nuclear fuel assembly comprising a plurality of sub-assembly fuel modules which are joined together to form an integral fuel assembly.
2. Description of the Prior Art
Prior art nuclear fuel assemblies are typically formed from a plurality of fuel rods supported in a skeletal assembly comprising a top and bottom nozzle, guide tube thimbles, and an instrument tube thimble extending between the top and bottom nozzles. A plurality of grid assemblies are axially spaced along the guide and instrument tube thimbles for laterally supporting the fuel rods.
The fuel rods typically comprise uranium dioxide fuel pellets contained within Zircaloy tubing which is plugged and sealed at its ends to encapsulate the fuel.
The bottom nozzle serves as a lower support structural element for the fuel assembly and directs coolant flow distribution to and through the fuel assembly. A typical bottom nozzle is fabricated from stainless steel and comprises a perforated plate having downwardly extending corner angle legs which form a plenum for the inlet coolant flow. The perforated plate prevents "downward" ejection of fuel rods from the fuel assembly. As used herein, directional words such as "up", "down", "top", "bottom", "left" and "right" are for establishing conventions only and should not be construed to limit the invention in any way.
Axial or holddown loads imposed on the fuel assembly as well as the weight of the fuel assembly are transmitted through the bottom nozzle to a lower core plate which is a well known part of the reactor internals.
The top nozzle functions as the upper structural element of the fuel assembly. A top nozzle may typically comprise a perforated plate having holes to accept the guide and instrument tube thimbles and perforations for permitting the flow of coolant "upward", out of the fuel assembly.
The guide tube thimbles are structural members which also provide channels for control rods or the like. A typical guide tube is fabricated from Zircaloy tubing. The lower end of the guide thimble is typically attached to the bottom nozzle by means of an end plug which is fastened to the bottom nozzle by a weld locked screw or the like. The fuel assembly will typically have a centrally located instrumentation tube thimble held between the top and bottom nozzles for accommodating in-core neutron detectors or the like.
As alluded to above, the fuel rods are typically provided with lateral support along their length by grid assemblies which maintain proper lateral spacing between fuel rods. Typically, each fuel rod will be supported by four or six contact points at each grid location by a combination of support dimples and springs. Prior art grid assemblies are known which are fabricated from individual slotted straps, interlocked and brazed in an egg-crate arrangement. Grid straps are usually fabricated from Inconel because of its corrosion resistance and high strength. However, Zircaloy grid straps have also been used to enhance neutron economy within the core.
The grid assemblies are axially spaced along the guide tube thimbles and are captured by bulge joints or the like formed in the guide tube thimbles at the grid assembly locations, or by other fasteners, in order to create an integrated skeletal structure in which the fuel rods can be inserted.
In U.S. Pat. No. 4,620,960 an apparatus is disclosed for mounting a top nozzle on a fuel assembly in a manner whereby it may be readily removed in order to allow access to and removal of a failed rod.
In commonly assigned, co-pending Ser. No. 564,049, filed Dec. 21, 1983 a fuel assembly is described in which partial grid assembly structures are used to laterally support or deflect coolant flow through only a portion of the fuel rods within the fuel assembly.
However, known prior art fuel assembly designs fail to provide for modular fuel assembly reconstitution which in turn facilitates repair of damaged fuel assemblies by permitting quick replacement of a prefabricated portion of the assembly. For example, if a corner of a grid strap of a grid assembly of a prior art fuel assembly is damaged either during transport or refueling operations, it may be necessary to scrap for salvage the entire fuel assembly.
Further, it is well known that at the periphery of a reactor core, specifically near a reflector or a baffle plate, the power generated is very low, and the fuel in the fuel assemblies directly opposite a reflector or baffle plate tends to deplete very slowly, thereby creating a burnup skew within both the reactor core and the fuel assembly. Since, during refueling, it is common practice to load the majority of feed fuel assemblies into peripheral core positions, fuel assemblies previously occupying those positions are usually repositioned at inboard core locations. Prior art fuel assemblies, when repositioned from peripheral to inboard locations, tend to lead the core in power peaking factors. In addition, core symmetry considerations restrict the number of fuel shuffling options available. When a large, prior art fuel assembly, for example a 19.times.19 lattice design, is relocated within a reactor core during refueling operations, it may be difficult to position such relatively unburned fuel at locations where it will avoid any power peaking concerns.
Further, large prior art fuel assemblies such as a 19.times.19 lattice, do not generally have the fuel enrichment within the fuel assembly tailored to avoid the power peaking and core symmetry considerations discussed above.
In addition, during refueling, it is beneficial to be able to inspect as many fuel rods as possible to check for rod bow, surface appearance, etc. With present fuel assemblies the most straightforward and commonly employed inspection method comprises a visual examination. A simple visual inspection is, however, limited to only the peripheral rods of a prior art fuel assembly. In other words, unless the fuel assembly design permits quick disassembly, interior rods cannot be visually inspected in a simple and expedient manner. In addition, with conventional large fuel assemblies, whenever it is desired to incorporate an advanced product into a demonstration program, an entire fuel assembly is at risk since the demonstration fuel assembly must be the same size as the other fuel assemblies presently in the core in order to properly meld into the core geometry.